loss of power at high speed "75 r90/6

[SoFloPro]

1975 r90/6

i was recently on a trip (miami-orlando) on the way back i felt a loss power at high speed, kinda like the clutch was actuated. i recently changed the throttle cables, but overall the motor felt strong on the way to orlando, but on the way back it was a lil different. i noticed the loss of power at speed (80mph) i was accelerating to pass and the rpms just started goin up. i backed off the throttle and slowly got up but still if i pushed it would happen again. what could be the cause of this issue?

( yeah i know i was speeding a bit... on the way to a wedding, no not mine..)

[Bill Burke]

This could be a number of things. Here's a start:
1. Tank vent problem. Open fuel cap when it cuts out and see if power is restored.
2. Battery weak. Weak battery can lead to loss of spark at speed.
3. Torn or holed diaphragm in a carb.
4. Fuel flow issue (clogged screens inside tank, or floats need adjustment to let more fuel in bowls).
5. Ignition problem. Do a "cap to cap" resistance test with an ohmmeter. (Testing from one spark plug cap to the other). Total resistance should be in the neighborhood of 20K ohms. If it's much higher or much lower, then it indicates a bad cap, a bad plug wire or a bad coil.

Others will have suggestions too.

[James.A]

If you experienced a situation where the motor speed (RPM) increased, but the vehicle speed did not, that sounds like the clutch was slipping. Is the clutch release mechanism correctly adjusted? There should be a slight bit of free-play at the handle when fully released.

[RRDavis]

The title of your post mentions "lack of power".

The symptoms you state seem to imply that the power is not being transferred from the motor to rear tire.

It's possible that your clutch is slipping.

[DennisDarrow]

Clutch Slipping.............Screw the adjusting nut at the handgrip in all the way.............Do you have 2mm of freeplay down at the arm behind the transmission where the other end of the cable fits?............Just grasp the arm where the cable fits and see if it moves a couple of MM.........thats about the thickness of a nickle...........Dennis

[amiles]

Hard acceleration in fifth gear, resulting in rpm climbing but speed not is a classic indication of a slipping clutch on your motorcycle. Check the adjustment as others have suggested.

If no relief Grab your Wallet, probably +$500. 00 or so for parts as I recall.

I had this problem and was able to limp along for a while until I finally fixed it.

[yaktackler]

I have the exact same thing happening on my 76 R90. For me the problem is intermittent. I have only ever felt the slip of the clutch and the increase in rpms in 5th gear and at high speed. It will happen at high speeds several times on a long ride typically under hard acceleration and then will not feel it for a week. Thanks for the posting.

[Boxerkuh]

You said that you just installed new throttle cables before the trip. On occassion, but especially when new, you might need to re-adjust the throttle cables. After the first 500 to 1000 miles it is not a bad idea. It may be just that simple... my food for thought... :eat :drink

[roncooper]

If you continue to run it with the clutch slipping the flywheel will be damaged because of heat.

[lkchris]

Loss of power at high speed is almost always torn carb diaphragms.

Lots of surging.

Interruption of fuel flow is more dramatic, i.e. the motor quits.

If clutch slips, it will do it on acceleration, too.

Clutch slip will be accompanied by increase in rpms.

[Na Cl K9]

1975 r90/6...i was accelerating to pass and the rpms just started goin up. i backed off the throttle and slowly got up but still if i pushed it would happen again. what could be the cause of this issue? It sounds to me like one of the following: Transmission oil migrating to the clutch disc from a bad transmission input shaft seal; a dirty or worn (out of spec – too thin) clutch disc; or a weak diaphragm spring. A rusty or warped pressure ring will cause a bit of slippage too. By all means, check your clutch cable adjustment and set the proper free-play using the transmission adjustment bolt, not the clutch lever adjuster… If you must eventually pull the transmission to see what is wrong, an oil soaked clutch disc will be easy to smell, will be a sticky mess and will probably need to be replaced. The diaphragm spring height and clutch thickness will need to be measured. BMW has a tool for checking a pressure ring for warp but a simple straight-edge metal rule will do for a quick test.